1. Field of the Invention
The present invention relates to a fresnel lens having a shading layer and a display device such as a liquid crystal display device including magnifying fresnel lenses.
2. Description of the Related Art
Liquid crystal display devices can have relatively thin structures and have been used for many applications. Recently, projection type liquid crystal display devices having larger screens have been developed. A typical projection type liquid crystal display device includes a projection lens which projects a magnified image onto a screen. Also, optical elements other than a projection lens can be used for magnifying an image.
For example, Japanese Unexamined Patent Publication (Kokai) No. 5-188340 discloses a projection type liquid crystal display device including liquid crystal display panels, fresnel lenses for magnifying images produced by liquid crystal display panels, and a screen. In this case, the liquid crystal display device also includes arrays of convergently transmissive elements, and a screen. Each of the arrays of convergently transmissive elements is adapted to form an erect and real image having an identical size to an object, and each of the fresnel lenses serves to magnify the image from the array of convergently transmissive elements.
The convergently transmissive elements are made from plastic or glass in the form of transparent rods having the diameter of 1 mm to 2 mm, so that refractive index changes in each of the transparent rods in the radial direction thereof. By appropriately selecting the length and the distribution of refractive index thereof, it is possible to use each of the convergently transmissive elements so that it can form an erect and real image having an identical size to an object. A plurality of convergently transmissive elements are arranged in a close relationship to each other with the end surfaces of the elements arranged in a line or in a plane, to thereby form a row or an array of convergently transmissive elements. The array of convergently transmissive elements can be used as an imaging device for producing an erect and real image having an identical size to an object. The imaging device using the array of convergently transmissive elements has advantages, compared with a usual spherical lens, in that a focal distance is very short and an optical performance is uniform in the line or plane so that an adjustment of the distance between the lenses is not necessary.
However, when the array of convergently transmissive elements is used as the imaging device, it is not possible to change a magnification of the image although it is possible for individual convergently transmissive elements to be changed in magnification by changing the length of the elements. This is because magnified images produced by the individual convergently transmissive elements are inconsistently superposed, one on another, in the array and a normal image cannot be formed. Therefore, the array of convergently transmissive elements can be used only as a full size imaging device, and it is necessary to provide a magnifying means in addition to the array of convergently transmissive elements.
Japanese Examined Patent publications (Kokoku) No. 58-33526 and No. 61-12249 disclose an imaging device including an array of convergently transmissive elements and a convex lens or a concave lens as a magnifying means which is arranged on the inlet side or on the outlet side of the array of convergently transmissive elements. The convex lens or the concave lens can be of a single lens or a composite lens of a plurality of lens components to realize a desired magnification. However, when this imaging device is used with a magnifying device in a liquid crystal display device, a problem arises in that resolving power of the lens changes from the central portion to the peripheral region.
It has been found that a good image is obtained if the resolving power MTF is greater than 50 percent under the condition of 4 (1 p/mm) i.e., 4 pairs of white and black spots per millimeter. However, it is generally difficult to establish an image having resolving power MTF greater than 50 percent in the above described prior art. It is necessary that light passes through the peripheral region of the liquid crystal display panel at an angle of approximately 10 degrees relative to the normal line of the liquid crystal display panel in order to ensure resolving power MTF greater than 50 percent. The smaller the angle at the peripheral region is, the smaller the magnification of the device is. As a result, it is not possible to realize a liquid crystal display device having a thin structure if a convex lens or a concave lens is used with an array of convergently transmissive elements, although the array of convergently transmissive elements by itself can provide a liquid crystal display device having a thin structure.
Accordingly, a magnifying element is desired which can be used with an array of convergently transmissive elements and which can realize a liquid crystal display device having a thin structure. The use of a fresnel lens with an array of convergently transmissive elements is disclosed in the above described Japanese Unexamined Patent Publication (Kokai) No. 5-188340, but the manner in which the fresnel lens is used is not described in this prior art. The inventors have recently found that a good result is obtained if a fresnel lens is used as a magnifying element.
Further, in a liquid crystal display device, there is a problem that brightness of an image on a peripheral region of the screen is reduced relative to the brightness of the image on the central region of the screen.